Yes, in most cases a homeowner can dig a French drain themselves to fix a soggy or waterlogged lawn — it's one of the more achievable DIY drainage projects. That said, there are a few important things to sort out before you pick up a spade: where the water will discharge to, whether you need a building consent, and whether any underground services are in the way.
What Is a French Drain?
A French drain (also called a rubble drain or agricultural drain) is a simple trench filled with gravel and a perforated pipe that collects excess groundwater and redirects it away from a problem area. Water seeps into the gravel, enters the pipe, and flows by gravity to a safe discharge point — typically a stormwater drain, soakage pit, or a lower part of your property.
Planning Before You Dig
Good planning is the difference between a drain that works and one that causes new problems.
Find Your Underground Services
Before any digging, call Dial Before You Dig (0800 248 344) or visit their website to identify buried power cables, gas lines, water pipes, and telecoms. Striking a service line is dangerous and costly. Your local council may also have maps of stormwater and wastewater lines.
Work Out Where the Water Will Go
Your French drain needs a legal and effective discharge point. Common options include:
- Connecting to your property's existing stormwater system
- A soakage pit or soakage trench in a suitable area of your section
- A lower-lying area of your own property where water can disperse naturally
You generally cannot discharge stormwater onto a neighbour's property or into the wastewater (sewer) system. Check with your local council about what's permitted in your area, as rules vary.
Do You Need a Building Consent?
Many simple drainage works on private property fall under Schedule 1 of the Building Act 2004 as exempt building work — but this is not guaranteed. Factors like the scale of the work, proximity to boundaries, and connection to council infrastructure can affect this. Always confirm with your local council before starting. You can find guidance at MBIE Building Performance.
What You'll Need
| Item | Notes |
|---|---|
| Spade and/or mattock | For hand digging; a hired mini-excavator speeds things up |
| Perforated ag pipe (100 mm) | Widely available at Bunnings or Mitre 10 |
| Drainage metal / coarse gravel | 20–40 mm clean chip works well |
| Geotextile filter fabric | Wraps the gravel to prevent silt blocking the pipe |
| Spirit level or laser level | To check fall/gradient |
| Wheelbarrow and tamper | For backfilling |
Bunnings and Mitre 10 both stock perforated ag pipe, drainage chip, and filter fabric suitable for this type of project.
Step-by-Step: Digging Your French Drain
1. Mark Out the Trench Route
Plan a route from the wet area to your discharge point. The trench should follow the natural slope of your land where possible.
2. Dig the Trench
A typical French drain trench is 300–500 mm wide and 500–900 mm deep, depending on how severe the waterlogging is. You need a consistent fall of at least 1:100 (1 cm drop per metre of run) toward the outlet — more fall is better. A hired mini-excavator makes this much faster if you have a long run or hard ground.
3. Line With Filter Fabric
Lay geotextile filter fabric into the trench, leaving enough overlap to fold back over the top later. This stops fine soil particles from migrating into the gravel and blocking the pipe over time.
4. Add a Gravel Bed
Pour in a 100–150 mm layer of clean drainage chip as a bed for the pipe.
5. Lay the Ag Pipe
Place the perforated ag pipe on the gravel bed with the holes facing down (this is the standard practice — it allows water to rise up into the pipe from below). Connect sections with couplings and ensure the pipe runs continuously to the outlet.
6. Backfill With Gravel
Fill the trench with drainage chip to within about 150–200 mm of the surface, then fold the filter fabric over the top of the gravel.
7. Top Up and Reinstate
Finish with topsoil and re-grass, or use a layer of decorative stone if the drain runs through a garden bed.
Common Mistakes to Avoid
- Insufficient fall — water won't flow if the gradient is too flat
- Skipping filter fabric — the drain will silt up within a few years
- No outlet — the water has to go somewhere; a dead-end drain just moves the problem
- Digging too shallow — in heavy clay soils, you may need to go deeper to reach a more permeable layer
When to Call a Professional
Consider hiring a qualified excavator operator or drainage contractor if:
- The waterlogging is severe or affects a large area
- You need to connect to the council stormwater network (this may require a licensed drainlayer under the Plumbers Gasfitters & Drainlayers Board rules)
- The ground is unstable, very rocky, or you're working near foundations or retaining walls
- You're unsure about the cause of the wet area (it could be a leaking pipe rather than groundwater)
- You want the work done to a standard that could support a future LIM or building consent
A qualified local excavator or drainage specialist can be found on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
